The composite spectrum of Japanese stereo TV broadcast audio comprises three channels, each of which occupies a distinct frequency band. The three channels are the MAIN, SUB, and CONTROL channels, which are also referred to as the monaural, stereo and control channels. The control channel is amplitude modulated (AM) and contains information that is utilized to inform a receiver whether the broadcast is in monaural, stereo or dual mono mode. When the AM carrier is absent, the broadcast is in mono, and when the AM carrier is present, the AM sidebands frequency will be at either 982.5 Hz or 922.5 Hz for stereo and dual mono broadcast, respectively The SUB channel is a frequency modulated (FM) channel, and the MAIN channel is not modulated. Depending on a frequency and amplitude of an input audio signal, the SUB channel may have sidebands frequency contents that leak into the adjacent AM control channel. The leakage into the adjacent AM control channel is typically in the form of a single-sided peak of amplitude A, which may be mathematically decomposed into two double-sided pairs, each having an amplitude A/2. One of the double-sided pairs AM and the other double-sided pair is phase modulated (PM).
A clean control channel, one without leakage, would have AM sidebands at 922.5 Hz on both sides of the center carrier frequency if the broadcast is in dual mono mode, or at 982.5 Hz on both sides of the center carrier frequency if the broadcast is in stereo mode. A decoder comprising a phase-locked loop (PLL) may lock onto the AM carrier, and an AM demodulator is utilized to recover the AM sideband at a frequency of either 922.5 Hz or 982.5 Hz. In cases where there is no leakage, after band pass filtering, the resulting two envelopes at these two frequencies may be compared with each other. As a result of this comparison, a decoder may be able to discriminate between broadcast stereo mode and dual-mono mode. However, when leakage from the SUB channel is sufficiently large or non-negligible, and is also at the incorrect frequency of either 982.5 Hz or 922.5 Hz around the AM carrier, the leakage contributes an AM component of amplitude A/2 that is present at the output of the AM demodulator. As a result of this leakage, the decoder might incorrectly decode the actual broadcast mode. For example, if a broadcast is in stereo mode, then a receiver may incorrectly mis-detect it as a dual-mono mode. Similarly, if the broadcast is in dual-mono mode, then a receiver may incorrectly mis-detect it as stereo mode. Additionally, in certain border instances, the receiver may be unable to determine whether the broadcast is in stereo mode or dual-mono mode and may keep switching back and forth between the two modes.
FIG. 1a is a graph illustrating the spectrum 100 of a Japanese TV broadcast audio signal with the second FM sideband leaking into the control channel. Referring to FIG. 1a, there is shown a frequency modulated (FM) channel 102 and an amplitude modulated (AM) control channel 104 of the spectrum 100.
FIG. 1b is a more detailed graph illustrating a zoomed portion of the AM control channel for the spectrum of FIG. 1a for DualMono. Referring to FIG. 1b, there is illustrated an AM carrier 110 located at 3.5 fH Hz, where fH is the horizontal frequency at 15,734 Hz. To the right of the AM carrier 110 is an upper AM sideband signal 112a and to the left of the AM carrier 110 is a lower AM sideband signal 112b. In the case of a dual-mono signal as shown here in FIG. 1b, the upper AM sideband signal 112a is located at 922.5 Hz above the AM carrier 110, and the lower AM sideband signal 112b is located at 922.5 Hz below the AM carrier 110. In the case of a stereo signal, the upper AM sideband signal 112a is located at 982.5 Hz above the AM carrier 110, and the lower AM sideband signal 112b is located at 982.5 Hz below the AM carrier 110. FIG. 1b also illustrates a leakage signal 114 that has leaked into the AM channel. In this regard, the leakage signal 114, is a single sideband signal, which is located at 982.5 Hz below the AM carrier 110.
In operation, for example, during dual-mono mode, the leakage signal 114, which is located at 982.5 Hz is located at the same frequency as would a stereo AM sideband signal. Accordingly, this could cause a receiver to mis-detect the dual-mono broadcast mode, and incorrectly characterize it as a stereo mode.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.